'65 Dodge Motor Decision

I have a 1965 Dodge 3/4 Ton 4x4 Flatbed with one ton axles and a military two speed manual transmission. Presently, it has a tired 318 Y block in it. I am debating rebuilding that motor or putting in something different. I want to be able to pull a 20 foot fifth wheel equipment trailer with it at 55 miles an hour on the highway with a thousand pound load(raw lumber) on it. Am I dreaming? Should I give up on this rig and just save up for something newer? What's your opinion?

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